This invention is invention is concerned with soldering fluxes particularly with such fluxes which are useful for a variety of soldering applications, such as soldering the radiators of motor vehicles and heat exchanger assemblies.
Various soldering procedures are used in the manufacture of car radiators and heat exchanger assemblies. One of these is the baking procedure which involves coating tubes with solder, clamping an assembly of the tubes and of other radiator components in a jig, spraying with flux or dipping the assembly in flux and baking the assembly in an oven. This procedure requires the assembly to remain in the oven at elevated temperature usually for several minutes. The oven temperature may be anything from 250.degree. to 470.degree. C. but is more likely to be in the range 280.degree. to 400.degree. C. During this procedure the solder will melt and flow into the crevices in the joints of the assembly thereby providing the necessary bonding. Another soldering process used in these industries is end-dipping and this also involves exposure to high temperature.
The flux used in these soldering processes must therefore have a sufficiently long high temperature life to ensure that it will provide sufficient protection to the surfaces to be joined until the radiator has attained a temperature sufficient to melt the solder and thereby permit bonding. Other desirable properties of fluxes are that they should produce low levels of char and residue and of course have a good fluxing efficiency.
Examples of fluxes which have previously been developed for such uses are described in British Patents Nos. 1517116 and 1553069. The earlier of these discloses a soldering flux which comprises an aqueous solution containing a mixture of 25 to 75% by weight of a hydrobromide of an aliphatic hydroxyamine and 75 to 25% by weight of a hydrobromide of an aliphatic amine. The latter is a development of this which describes and claims a flux which comprises an aqueous solution containing a mixture of diethanolamine hydrobromide and ethylamine hydrobromide together with ammonium bromide.
After soldering using these fluxes, the used flux will contain an accumulation of metal ions including copper II and zinc II ions. The used flux is usually chemically treated with lime or sodium hydroxide until a pH of about 9 is reached, which causes the metal ions to precipitate as insoluble hydroxides. The metal free waste is then discharged to drain. The treatment with alkali (e.g. lime) is carried out to remove metal ions for environmental reasons.
The applicant has now found that when used flux from commercial fluxes such as those described above is made alkaline a significant proportion of metal ions may be retained in solution rather than precipitated out as insoluble hydroxide. This has been found to occur with fluxes containing aliphatic amines of the type R.sub.2 N(CR.sub.2).sub.n NR.sub.2 or R.sub.2 N(CR.sub.2).sub.n OH in which n is 2 to 4 and R is hydrogen, methyl or ethyl. This presents an environmental problem because copper and zinc ions which were supposed to have been removed have not been and are therefore being discharged to drain.